Engineers strive to keep Japan's reactors under control

Engineers are still trying to bring the situation at the Fukushima Daiichi nuclear plant under control after Friday’s earthquake and resulting tsunami crippled several back-up measures leading to a system-wide cooling problem.

The plant experienced three major hydrogen-based explosions in reactors 1, 3, and then 2. Most recently there has been a fire confirmed at the site of reactor 4, which although shut down for maintenance at the time of the earthquake, houses spent fuel rods.

While radiation has leaked locally, the type of radionuclides involved, the extent of coverage and what effect this may have on human health remains unclear.

As reported previously by The Engineer, reactors 1, 2 and 3, which were operational at the time of the earthquake on Friday, were immediately shut after sensors at the plant detected ground acceleration. Reactors 4, 5 and 6 were not operational at the time, but some housed spent fuel rods.

Although the fission reaction was stopped immediately in reactors 1–3 by inserting control rods between the fuel assemblies, fuel carries on emitting thermal heat and radioactive decay — and this is where the problems have arisen.

Systems failure

Multiple back-up systems intended to cool the reactors failed after extensive damage from the tsunami, which swept away the power lines and all external power to the plant.

Diesel generators were able to provide power back-up for the cooling system for around an hour, but ultimately failed due to prior damage. Reactor Core Isolation Cooling (RCIC) pumps were also used but the battery-supplied control valves lost DC power after the prolonged use.

Engineers were forced to inject seawater into the reactor pressure vessel to continue the cooling process. As a consequence of efforts to cool the reactors, pressure built up inside the vessels, which had to be vented causing the explosions, although Prof Mike Reeks, from the School of Mechanical and Systems Engineering at Newcastle University, said this was also part of documented procedure.

‘This venting is controlled in the sense that they know they have to release some pressure from the reactor and what people haven’t pointed out is that they vent through filters, so that takes a lot of the radioactive particles and also some of the fission product gases,’ he said.

Paying tribute to the engineers at the Fukushima Daiichi plant, Reeks added: ‘This is what is known as a station blackout; it’s something that people studying severe accidents look at. They’re doing a great job under very unfortunate circumstances… they’ve followed the right procedures.’

International review

Several countries, notably Germany, have made announcements to review their nuclear plants.

In the UK, energy secretary Chris Huhne has asked chief nuclear inspector Dr Mike Weightman for a thorough report on the implications of the situation in Japan and lessons to be learned from it.

Prof Neil Hyatt of the Department of Materials Science and Engineering at Sheffield University said: ‘The current Japanese nuclear emergency should not cast a shadow over building new nuclear stations in the UK.

‘The proposed new fleet of reactors do not require a diesel generator to keep coolant flowing through the core, which failed in the Fukushima reactors.’

Readers' comments
(5)

Anonymous | 16 Mar 2011 2:34 pm

Again the media seems to have hyped up the whole thing. No arguing, Japan has problems. Why do we in europe over react. Do we suffer from Tsunamis (No). Do we sit a Tectonic plate with the potential for huge slips (No). Do we have a weak and fickle governement who over react to a minority of extremists (who would have us all living in the middle ages if they got there way, and would then be the first to complain) Yes. Why ignore the specialist and the scientist. It is one of the best ways of solving our electricity supply shortages. Alternatively let's open our mines up and build a new generation of coal fired, effficient baseline generators. instead of wasting our precious oil and gas resources (forgive me it is sickening) To my surprise Germany seems to be more fickle than us.

That "minority of extremists" would not "have us living in the middle ages if they got their way" any more than your minority of "nuclear-worshipping extremists" would have us living as radioactive mutants with reptilian skin, 5 eyes and humps!!Prudence, common sense, and a concern for these extraordinary situations somehow unimagined in engineers' previous assessments and planning dictate that we weigh advantages and disadvantages, costs as well as benefits.We have abundant natural energy sources that have hardly been mined or explored in comparison to these technologically-heavy and potentially environmentally-devastating (but very profitable to the operating corporations) nuclear and petroleum/coal-based systems. Before we subject our planet and our people to more duress gene and health-wise, why can we not devote the same pioneering efforts we devoted to the space race to clean alternative energy systems based instead on abundant solar and wind?PS- Iceland is seismically active and has the potential to create a tsunami in the N. Atlantic with a lesser distance to travel to Europe.

I wonder why we cannot build the reactors and store fuel rods in underground cylinders which could then be imploded with earth if full blown meltdown is occuring to minimize release of radioactive ions into the atmosphere. Is this possible?

Tribute indeed to those on the front line of this disaster, facing an unremmiting battle against a terrible deadly force of nature. No doubt history will have the last say on the ultimate price man will eventually pay for his embrace of the nuclear genie but for now let our hearts be with the Japanese nation.

We may not have Tsunamis to cut off power and destroy back up diesel generators, but we do have Terrorists who'd like to do the same thing. The lesson from Fukushima is not that similar power plants are safe: it is that unexpected things happen & in combinations that were not planned for.The Fukushima reactors may have performed well beyond their design parameters, but it remains to be seen if they performed well enough.